In the production of hydrocarbons from subterranean reservoirs, the deposition of solid salts, or scale on surfaces, down hole tubing and equipment is a major production problem. The problem increases as the water-cut in the produced fluid increases. Scales are slightly soluble inorganic salts, such as calcium or barium sulfates or calcium carbonate, etc. These minerals are often found in the produced water of hydrocarbon recovery operations. Inhibition of scale formation is significantly more cost effective than a well cleanout and especially so when a threshold chemical treatment is used. Wells can be treated with scale inhibiting chemicals either as a batch job, with continuous treatment down the annulus or through a feed string, or by a squeeze treatment. A squeeze treatment is a common oil field practice for treating subterranean formations to inhibit scale formation. In a squeeze treatment, a scale inhibitor treatment is injected or squeezed into the reservoir formation. The scale inhibitor forced into the formation is either absorbed by the reservoir mineral, becomes phase trapped or precipitates in the formation. When the well is placed back into production, the inhibitor is slowly produced back into the well bore with the produced water to inhibit scale formation.
The most commonly used classes of inhibitor compounds are phosphonic acid salts, phosphate ester salts and polymeric (generally low molecular weight acrylic acid salts or modified polyacrylamide and copolymers thereof). Inorganic phosphates are also used but to a lesser extent. In squeeze treatments, the selection of the inhibitor used should be based upon which product gives the longest squeeze life, assuming inhibition and temperature stability requirements are satisfied.
A procedure called "precipitation squeeze" has been developed. In this process, the scale inhibitor reacts or is reacted to form an insoluble salt which precipitates in the pores of the formation rock. For example, see the process described in U.S. Pat. No. 4,860,829 in which a phosphonate scale inhibitor and a calcium chelate are employed as a "precipitation squeeze" treatment. The use of a phosphinico polycarboxylate in a precipitation squeeze treatment was disclosed in Results of Field Tests With a New Extended Squeeze Life Scale Inhibitor, a paper presented by J. E. Pardue at the 1992 NACE annual conference and corrosion show.